In a selective call radio communication system having a forward channel for transmitting digital messages to selective call transceivers from a system controller and receiving digital messages and responses at the system controller from the selective call transceivers in one or more reverse channels, a known means of organizing the responses in the reverse channels is to schedule the responses, using information transmitted in the forward channel to the selective call transceivers to perform the scheduling. This is a general approach used in a variety of radio systems.
A first example of such a system is a single frequency half duplex system having one or more transmitters for transmitting the digital messages at a radio carrier frequency from the system controller and one or more fixed receivers for receiving the messages and responses from the selective call transceivers on the same radio carrier frequency. The transmissions from the system controller are typically suspended to allow responses from the selective call transceivers. The responses may be transmitted from the selective call transceivers using a random or semi-random method, or they may be organized by information transmitted from the system controller. The organized method works particularly well in a system having a large percentage of acknowledgments or other demand type of responses. The demand type of responses can be scheduled during the transmission from the system controller which generates the demand responses. The selective call transceivers can include in random responses information indicating to the system controller that additional messages are being held within the selective call transceivers, awaiting scheduled transmission to the system controller. With this information, the system controller can schedule the held messages for transmission on the reverse channel.
A second example of a system using scheduled responses in a reverse channel is a duplex system having one or more transmitters for transmitting the digital messages at a radio carrier frequency from the system controller and one or more fixed receivers for receiving the messages and responses at a second radio carrier frequency from the selective call transceivers. In this system, the transmissions from the system controller do not need to be stopped to allow responses from the selective call transceivers. However, the responses may be organized, as in the case of the first example, by information transmitted from the system controller.
In the case of both examples, some generic parameters of the scheduled reverse channel transmissions are used by the fixed receivers for recovering the reverse channel information from the transmissions made by the selective call transceivers. For example, predetermination of the modulation and bit rate to be received in the reverse channel is typically needed by the fixed receivers to properly recover the reverse channel information. Also, in a half duplex system, the fixed receiver may need a predetermination of when the radio frequency is being used for the reverse channel, to avoid misinterpreting information that is actually forward channel information as reverse channel information. The message and response information communicated within systems such as given in the examples described above may be transmitted in packets of one or more predetermined fixed lengths. A first packet of a group of packets, or a single packet, transmitted by a selective call transceiver typically includes a packet synchronization portion at the start of the packet, which provides bit synchronization and identifies the beginning of a second portion of the packet consisting of data words. Packets in a group of packets, which are not the first packet of the group, may derive their bit and word synchronization from the first packet in the group and therefore not include a synchronization portion.
Using the predetermined modulation and reverse channel timing information, a fixed receiver can recover most of such packets transmitted in the reverse channel by the selective call transceivers, when the packets are received within radio signals having a signal strength above a minimum threshold. However, there are circumstances which may prevent the fixed receivers from recovering some of these radio packets. These circumstances arise when the fixed receiver's synchronization pattern detector is falsed by noise, resulting in a false detection of a synchronization pattern, further resulting in a false indication of the beginning of the word portion of a packet. When this happens, the fixed receiver can miss the decoding of the synchronization pattern and code words for a transmitted packet because the fixed receiver is incorrectly processing data symbols from the transmitted packet which are out of word synchronization sequence, due to the false detection of a synchronization pattern prior to the actual start of the transmitted packet. As a result of such false synchronization detections, packets and groups of packets are not decoded that otherwise could be, and the message throughput and message sensitivity performance characteristics are therefore not as good as they otherwise could be.
Thus, what is needed is a method to improve the message sensitivity and message throughput performance characteristics in digital radio communication systems having a forward and reverse channel and using forward channel scheduling for messages and responses transmitted in the reverse channel by one or more selective call transceivers